US6222321B1 - Plasma generator pulsed direct current supply in a bridge configuration - Google Patents

Abstract

Current controlled power sources are disclosed that are capable of generating currents in low resistance, high temperature plasmas that are regulated to prevent the generation of excessive currents in the plasma. Current reversing switches are provided that control the flow of a direct current in a plasma chamber between various electrodes. Multiple power sources are provided in association with shunt switches for delivering a plurality of sources of direct current in various directions between electrodes in a plasma chamber. Inductive impedance can be provided in switch paths to cause a source of direct current to flow through a plasma chamber in various directions between electrodes.

Description

This a continuation of application Ser. No. 08/646,616, filed May 8, 1996 now issued as U.S. Pat. No. 5,917,286 and hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention pertains generally to power supplies and more particularly to pulsed DC power supplies that are used for generating plasmas that are used in thin film processing techniques for etching, deposition, etc.

2. Definitions

Alternating polarities means a current flow at any particular point in a circuit or plasma that changes direction, or a voltage at any particular point in a circuit that changes magnitudes around any desired neutral voltage.

Current connections means locations or points in a circuit that are coupled to electrodes of a plasma chamber.

Current controlled power source means a power source that is capable of maintaining a substantially constant current for a wide range of load impedances and has a low amount of capacitively stored energy.

Current reversing switches means any desired arrangement of switches that are capable of causing current to flow in different directions at a preselected location in a circuit.

Direct current means current that has a substantially constant magnitude.

Direction means the course of the flow of current on a conductor in a circuit.

Generating means initiating and/or maintaining.

Inductor means an electrical component that is designed to store energy in a magnetic field.

Plasma means a state of matter in which electrons and ions in a gas discharge are separated but together form a neutral assembly.

Plasma chamber means a device in which plasmas can be generated.

Power source means a device that is capable of supplying electrical energy.

Predetermined positions means either an opened or closed position of a switch.

Pulsed direct current means a current that flows at a particular point in a circuit that has a first substantially constant magnitude during a first period of time, and then has at least one additional substantially constant magnitude that is different from the first substantially constant magnitude during at least one additional subsequent period of time, and may repeat.

Substantially constant supply means a substantially constant magnitude.

DESCRIPTION OF THE BACKGROUND

Plasma processing techniques have found wide-spread use in industry for commercial processes such as plasma vapor deposition, sputtering, etc. These processes have become particularly useful in thin film applications. To generate a plasma, a power supply creates an electric potential between a cathode and one or more anodes that are placed in a plasma chamber containing the gases that are used to form the plasma. When using these processes for deposition, the plasma acts upon the material of a target placed in the plasma chamber that normally comprises the cathode surface. Plasma ions cause target material to be dislodged from the cathode surface. The target materials are then deposited on a substrate deposition surface to form a thin film. The thin film may constitute material sputtered by the plasma from the target surface, as disclosed above, or may be the result of a reaction between the target material and some other element included in the plasma. The materials and elements involved, as well as the specific applications of the plasma processing techniques vary greatly. Applications may range from coating architectural glass to deposition of thin film layers on microchips, or deposition of aluminum layers on compact disks.

In the past, high frequency voltage sources have been used to generate a high electrical potential that produces a plasma within a plasma chamber. These high-frequency voltage sources are expensive to construct and maintain, as well as dangerous to operate. Additionally, if the deposition material is formed by reaction with an element in the plasma, and further, is electronically insulating, the anode in the chamber can be coated with the insulator; this deposit can then prevent the anode from performing its function of collecting the electrons released from the plasma during the deposition process.

To overcome these disadvantages, pulsed DC voltage sources have been employed such as disclosed in U.S. Pat. No. 5,303,139 issued Apr. 12, 1994 to Mark, which is specifically incorporated herein by reference for all that it discloses and teaches. Mark discloses a constant voltage pulsed power supply that has alternating pulse polarities. The advantages of such a constant voltage pulsed power supply over the AC power supplies are that they are less expensive, easier to connect and set up, and overcome the problem of coating the anode if used with two target units. That is, the process of reversing polarities allows the electrodes to alternately act as anode and cathode; the sputtering process that occurs during the cathode phase cleans off any deposited insulating material and permits uninhabited operation of the electrode as an anode during the anode phase. Additionally, the process of reversing polarities allows both electrodes to alternatively act as a cathode so that both electrode surfaces are capable of providing target material.

Despite the advantages that constant voltage pulsed power sources provide, problems exist with regard to generation of excessive currents and spark discharges in the plasma chamber. As part of this problem, it has been found that as the current through a plasma increases, the resistance of the plasma decreases in an exponential manner to almost zero. Small changes in the voltage level of a voltage power source result in large changes in the current. Consequently, excessive current increases can be generated from only very small changes in the voltage level, and a high degree of accuracy is required for controlling voltage controlled power supplies to prevent excessive current increases.

To exacerbate the problem, it has been found that various benefits accrue including increases in efficiency as the plasma temperature is increased in the plasma chamber. It is therefore desirable to produce high temperature plasmas that have low resistances and that require the use of power supplies that operate in a controlled manner to prevent the generation of excessive currents. The high power required to produce the desired plasma temperatures places extreme demands on the power supply. For example, the power handling capabilities of switches and other electrical components must be increased to meet such high power specifications.

SUMMARY OF THE INVENTION

The present invention overcomes the disadvantages the limitations of the prior art by providing a current controlled power supply that produces direct current pulses having alternating polarities to generate high temperature plasmas. A single power source, multiple power sources and/or multiple electrodes can be employed in accordance with the present invention.

The present invention therefore may comprise an apparatus for generating a pulsed direct current having alternating polarities to be applied to a plasma chamber to generate plasmas comprising, a power source that generates a substantially constant supply of direct current, current connections for delivering the pulsed-direct current to the plasma chamber, and current reversing switches having at least two pre-determined positions, the current reversing switches coupled to the power source and the current connections that cause the substantially constant supply of the direct current to flow in a first direction through the current connections whenever the current reversing switches are set in a first pre-determined position, and in a second direction through the current connections whenever the current reversing switches are set in a second pre-determined position.

The present invention may also comprise a method of generating a source of pulsed current having alternating polarities for use in generating a plasma comprising the steps of, generating a substantially constant supply of current from a current controlled power source, and switching current flow direction of the substantially constant supply of current to be supplied to said plasma using flow reversing switches that produce the source of pulsed current having alternating polarities for generating said plasma.

The present invention may also comprise a method for causing two substantially constant direct currents to flow in a plasma chamber comprising the steps of, generating a first substantially constant direct current using a first current controlled power source, generating a second substantially constant direct current using a second current controlled power source, connecting the first current controlled power source to the plasma chamber to cause the first substantially constant direct current to flow through the plasma chamber in a first direction during a first pre-determined period, and connecting the second current controlled power source to the plasma chamber to cause the second substantially constant direct current to flow through the plasma chamber in a second direction during a second pre-determined period.

The present invention may also comprise a circuit for generating a direct current that flows between a plurality of electrodes in a plasma chamber comprising, a current controlled power source that generates a substantially constant supply of the direct current, a switch connected between the current controlled power source and each electrode of the plurality of electrodes, and inductors coupled between the electrodes of the plasma chamber and a common return of the current controlled power source that cause the direct current to flow in the plasma chamber when at least one of the switches is open and at least one other is closed.

A first advantage of the present invention is that the current controlled power source provides a device for accurately controlling the amount of current that is applied to the plasma chamber despite changes in the resistance of the plasma. The switches that control the flow of current through the plasma chamber can also be utilized to shunt current. Multiple electrodes can be used in conjunction with either a single power source or multiple power sources to increase the deposition capabilities of the plasma chamber. Multiple electrodes allow for multiple target surfaces when each of the electrodes is sequentially employed as the cathode target surface.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph of current versus voltage illustrating the current/voltage characteristics of a plasma.

FIG. 2 is a schematic circuit diagram of a current source employed in accordance wit hone embodiment of the present invention.

FIG. 3 is a schematic circuit diagram of a first embodiment of the present invention that uses a single power source.

FIG. 4 is a graph of the current pulses that can be produced in the plasma by the embodiment of FIG.3.

FIG. 5 is a schematic illustration of an alternative arrangement of current pulses that can be produced in the plasma that provide a predetermined duty cycle.

FIG. 6 is a schematic illustration of another embodiment of the present invention that uses a single current controlled power source with three electrodes.

FIG. 7 is a graph of the voltage onelectrode64 when various switches are closed.

FIG. 8 is a graph of the voltage onelectrode68 when various switches are closed.

FIG. 9 is a graph of the voltage onelectrode66 when various switches are closed.

FIGS. 10-12 schematically illustrate various conditions in a plasma chamber for a first, second and third states of operation.

FIG. 13 is a graph of the voltage onelectrode64 for the fourth, fifth and sixth states of operation.

FIG. 14 is a graph of the voltage onelectrode68 for the fourth, fifth and sixth states of operation.

FIG. 15 is a graph of the voltages onelectrode66 for the fourth, fifth and sixth states of operation.

FIGS. 16-18 schematically illustrate various conditions of the plasma in the plasma chamber during the fourth, fifth and sixth states of operation.

FIG. 19 is a schematic illustration of an embodiment of the present invention utilizing a single power source and four electrodes.

FIG. 20 is a schematic illustration of another embodiment of the present invention that utilizes two current controlled power sources.

FIG. 21 is a schematic illustration of the current pulses that can be produced by the embodiment of FIG.20.

FIG. 22 is a schematic illustration of additional current pulses that can be produced by the embodiment of FIG.20.

FIG. 23 is a schematic circuit diagram of an embodiment of the present invention utilizing three current controlled power sources coupled to three electrodes.

FIG. 24 is a schematic circuit diagram of another embodiment of the present invention utilizing four current control power sources coupled to four electrodes.

FIG. 25 is a schematic circuit diagram of another embodiment of the present invention that utilizes a single power source and two switches.

FIG. 26 is a schematic circuit diagram of another embodiment of the present invention that utilizes a single power source with three switches and three electrodes.

FIG. 27 is a schematic circuit diagram of another embodiment of the present invention that utilizes a single power source, four switches and four electrodes.

FIG. 28 is a schematic circuit diagram of a voltage controlled power source that is coupled to three electrodes.

FIG. 29 is a schematic circuit diagram of a voltage controlled power source that is coupled to four electrodes.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION

FIG. 1 illustrates the advantages of driving the plasma generator with a current source through current reversing switches. FIG. 1 shows the current/voltage characteristics of a typical plasma chamber. As shown in FIG. 1, as the voltage increases, the current through the plasma chamber rises exponentially. As is readily apparent from FIG. 1, once the slope of the current versus voltage curve of FIG. 1 exceeds 45 degrees, it is better to control the power source with current rather than with voltage. Accordingly, at the operating point of a current i₁, a small change in voltage can produce a large change in current around i₁. For example, although a small change in voltage from v₁to v₂only produces a relatively small change in current from i₁to i₂an equally small change in voltage from v₁to v₃causes a very large change in current from i₁to i₃. Thus, a voltage source driving the plasma chamber is very susceptible to arc discharges as a result of minor variations or instability in the voltage.

However, if the plasma generating chamber is driven by acurrent source10 and theswitch configuration12 illustrated in FIG. 2, the current i₁may be easily controlled even though the voltage v₁may vary substantially about the operating point, because thecurrent source10 is capable of rapid voltage changes that may be required by rapid changes in the resistance ofplasma chamber14.

FIG. 2 illustrates a preferred embodiment of the invention which includes acurrent source10 driving a plasma chamber, as referred to above. In operation,current source10 drives a current18 that is applied toswitches20 and22 that are connected in a parallel configuration.Switches20 and22 can be alternately and substantially simultaneously closed to apply current to current connections ornodes24 and26 as illustrated in FIG.2. The current connections ornodes24 and26 are coupled toelectrodes28 and30 ofplasma chamber14.Node24 is connected to aswitch32, which is in turn connected to thecommon return36 ofcurrent source10. Similarly,node26 is connected to switch34, which is in turn connected to thecommon return36 ofcurrent source10.

Referring again to FIG. 2, theswitch configuration12 can be used as current reversing switches that operate in the following manner. In a first state of operation, switches20 and34 are closed and switches22 and32 are open. In this manner, current is caused to flow fromelectrode28 toelectrode30 inplasma chamber14. Hence, the direction of flow of the current in theplasma chamber14 is fromelectrode28 toelectrode30.

In a second state of operation, switches20 and34 are open and switches22 and32 are closed. This causes the current18 fromcurrent source10 to flow in theplasma chamber14 fromelectrode30 toelectrode28. Hence, the direction of flow of current in the second state of operation in theplasma chamber14 is fromelectrode30 toelectrode28. By operating theswitch configuration12 in this manner, pulses of direct current having alternating polarities can be generated in theplasma chamber14, as illustrated in FIG.4.

In a third state of operation, all four of theswitches20,22,34,32 ofswitch configuration12 can be closed so that no current flows throughplasma chamber14. It may be desirable to place theswitch configuration12 in this state when an arc discharge or a potential arc discharge is detected inplasma chamber14. Additionally, this state of operation wherein all the switches are closed during a preselected time period may be desirable to modify the duty cycle of the substantially constant supply of direct current18 being applied toplasma chamber14, such as illustrated in FIG.5.

FIG. 3 is a schematic circuit diagram of the manner in which the present invention is implemented with apower source38. As shown in FIG. 3,power source38 also includes aninductor40 which helps thepower source38 to function in a manner similar to an ideal current source, such as idealcurrent source10 shown in FIG.2.Power source38, in conjunction withinductor40, is constructed in a manner to approximate the operation of an ideal current source within the practical limits of operation using available components. For example, a sudden decrease in the resistance in theplasma chamber42 that results in an arc discharge betweenelectrodes44 and46 will cause an instantaneous shift in impedance toinductor40.Power source38 is designed to provide the desired impedance over longer durations.

In operation, the circuit of FIG. 3 operates in the same manner as described with regard to FIG.2. When switches48 and52 are closed simultaneously, current39 flows through theplasma chamber42 fromelectrode44 toelectrode46. In this case, the current39 is applied to current connection, ornode56 viaswitch48, whileelectrode46 is coupled to thecommon return60 ofpower source38 viacurrent connection58 andswitch52. In a similar manner, switches50 and54 can be closed simultaneously to cause current to be applied to current connection ornode58, and thecommon return60 to be coupled to current connection, ornode56, viaswitch54. In this case, direct current39 flows fromelectrode46 toelectrode44 inplasma chamber42. Hence, by alternately closing switches48,52 and50,54, the current flow through theplasma chamber42 produces direct current pulses having alternating polarities in theplasma chamber42, such as illustrated in FIG.4. Since the current39 is produced by apower source38 that generates a substantially constant supply of direct current, these pulses comprise pulsed direct current having alternating polarities in theplasma chamber42.

The device of FIG. 3 can also be operated in a third state of operation such as described with respect to FIG.2. In the same manner as described above, all four switches,48,50,52,54 can be closed so that direct current39 is shunted around theplasma chamber42. The direct current39 does not pass through theplasma chamber42. Theswitches48,50,52,54 comprise current reversing switches that are therefore capable of reversing the flow direction of current in theplasma chamber42, and also shunting the current39 so that no current flow throughplasma chamber42. Hence, in the third state of operation ofswitches48,50,52,54, pulses such as those illustrated in FIG. 5 can be generated so that a predetermined duty cycle of the operation ofplasma chamber42 can be produced. Also, in the same manner as described above with regard to FIG. 2, the third state of operation can be initiated when an arc discharge is detected, or the potential for an arc discharge is detected, to minimize damage caused in theplasma chamber42.

Thepower source38 of FIG. 3 is also designed so that only a small amount of capacitive storage is provided across its outputs. This allowspower source38 to function as nearly as possible as an ideal current supply.

FIG. 4 illustrates a source of pulsed direct current having alternating polarities that is applied to a plasma chamber to generate plasmas. As described above, switches48 and52 are closed whileswitches50 and54 are substantially simultaneously opened during a first state of operation that produces a pulse of direct current62 for a predetermined period in the plasma. At the end of such a predetermined period, a second state of operation is produced when switches48 and52 are opened whileswitches50 and54 are substantially simultaneously closed. During this second state of operation, a pulse of direct current64 is produced for a second predetermined period. This process can be repeated to produce a series of direct current pulses having alternating polarities such as illustrated bypulses66,68,70,72,74, and so on. FIG. 4 therefore illustrates the manner in which theswitches48,50,52 and54 can be operated alternatively between a first and second state to produce a series of direct current pulses having alternating polarities in a plasma chamber.

FIG. 5 illustrates the manner in which switches48,50,52 and54 of FIG. 3 can be alternatively operated in three different states. As shown in FIG. 5, switches48,50,52 and54 can be operated in a first state to produce a directcurrent pulse76. The switches can then be operated in a third state by closing all of theswitches48,50,52,54 to produceoutput78 during a second predetermined period. In the third state of operation, no current flows through theplasma chamber42 as shown atoutput78 of FIG.5. During a third predetermined period, the switches can be operated in a second state to produce acurrent pulse80 in theplasma chamber42. During a fourth predetermined period, the switches can again be operated in a third state to produceoutput82 during which no current flows through theplasma chamber42. This process can be repeated to produceoutputs84,86,88,90,92, and so on. The series of alternating polarity current pulses illustrated in FIG. 5 provide a predetermined operating duty cycle of theplasma chamber42 that is dependent upon the length of the operation of the switches in the third state.

FIG. 6 illustrates another embodiment of the present invention which utilizes asingle power source62.Power source62 generates a substantially constant supply of direct current74 that is applied to threeelectrodes64,66,68 in aplasma chamber70. As shown in FIG. 6,power source62 includes aninductor72 that allowspower source62 to approximate the operation of a current source that is capable of providing a substantially constant supply of direct current74. As shown in FIG. 6,power source62 has acurrent output73 that is coupled toparallel switches76,78 and80. Similarly, switches82,84,86 are connected in parallel tocommon return75 ofpower source62.Switch76 is coupled to connection ornode88, which is in turn connected toelectrode68.Switch78 is connected to connection ornode90, which is in turn connected toelectrode66.Switch80 is connected to connection ornode92, which is in turn connected toelectrode64.

The device of FIG. 6 has six different states of operation that are illustrated in FIGS. 7 through 18. FIGS. 7-9 and13-15 all illustrate voltage waveforms forelectrodes64,66 and68. These voltage waveforms illustrate the difference in voltage between these various electrodes and the plasma chamber, and also earth ground since the plasma chamber is usually connected to earth ground. The power sources, however, may float with respect to earthground and the plasma chamber.

FIG. 7 illustrates the voltage onelectrode64 during three states of operation. During the first state ofoperation94 thevoltage102 onelectrode64 is negative. The first state ofoperation94 occurs during the time period from times t1 to t2. Referring to FIG. 6, switches82,78 and76 are closed and switches80,84,86 are open during the first state ofoperation94. As can be seen from FIG. 6, the direct current74 is applied to node orconnections88 and90, that causes current to flow fromelectrodes66 and68 toelectrode64.

FIG. 10 illustrates the condition of theplasma chamber70 during the first state ofoperation94. As shown in FIG. 10,electrode64 comprises a cathode whileelectrodes66 and68 comprise anodes. Aplasma96 is generated proximate tocathode64, as illustrated in FIG.10.Ions98 are attracted tocathode64, whileelectrons100 fromplasma96 are attracted toanode66 andanode68.

FIG. 7 also illustrates thevoltage102 onelectrode64 during a second state ofoperation104 that occurs from times t2 to t3. As shown in FIG. 7, thevoltage102 onelectrodes64 is slightly positive during the second state ofoperation104.

Referring to FIG. 6, the second state of operation occurs when switches78,80 and86 are closed, and switches76,82,84 are open. When switches78 and80 are closed, current is applied toelectrodes64 and64 through nodes orconnections90 and92, respectively. Whenswitch86 is closed,electrode68 is coupled to thecommon return75 of thepower source62.

FIG. 11 illustrates the condition of operation of the plasma chamber during the second state ofoperation104. As shown in FIG. 11,electrode68 functions as a cathode, whileelectrode64 and66 function as anodes. Aplasma106 is generated proximate to thecathode68.Positive ions108 are attracted to thecathode68, while negative electrons are attached to anodes64 and66.

FIG. 7 additionally illustrates thevoltage102 onelectrode64 during a third state ofoperation110 that occurs from time t3 to t4. As shown in FIG. 7, thevoltage102 onelectrode64 during thethird state110 is slightly positive.

Referring to FIG. 6, the third state of operation occurs when switches76,80 and84 are closed and switches78,82 and86 are open. When switches76 and80 are closed, direct current74 is applied toelectrodes68 and64 vianodes88 and92, respectively. By closingswitch84,electrode66 is connected to thecommon return75 ofpower source62.

FIG. 12 illustrates the condition of theplasma chamber70 during the third state ofoperation110.Electrode66 functions as a cathode, whileelectrodes64 and68 function as anodes. Aplasma112 is generated proximate tocathode66.Plasma112 generatesions114 that are attracted tocathode66 and electrons that are attracted toanodes64 and68.

Referring again to FIG. 7, the first state of operation is again repeated between times t4 and t5, so that a negative voltage pulse is produced onelectrode64. Similarly, the second state ofoperation104 is repeated from times t5 and t6. These three states of operation can be repeated in the order shown, or any desired order of operation of theswitches76 through86. FIG. 8 illustrates thevoltages116 produced onelectrode68 during the three states of operation. As shown in FIG. 8, thevoltage116 onelectrode68 is positive during the first state ofoperation94, is negative during the second state ofoperation104, and is positive again during the third state ofoperation110. Thevoltages116 onelectrode68 are illustrated in FIGS. 10-12.

FIG. 9 illustrates thevoltages118 produced onelectrode66 during the three states of operation. As shown in FIG. 9, thevoltage118 onelectrode66 is positive during the first state ofoperation94 and the second state ofoperation104. Thevoltage118 onelectrode66 is negative during the third state ofoperation110. It is possible and reasonable to operate the system of FIG. 6 in only these three first states of operation or to operate additionally with states wherein more than one element at a time acts as a cathode.

FIGS. 13-15 illustrate the voltages on theelectrodes64,68 and66 during thefourth state120,fifth state122 andsixth state124.

FIG. 13 illustrates thevoltage126 onelectrode64 during thefourth state120,fifth state122 andsixth state124. As shown in FIG. 13, thevoltage126 onelectrode64 is negative during thefourth state120 andfifth state122. Thevoltage126 onelectrode64 is positive during thesixth state124. As also illustrated in FIG. 13, thevarious states120,122,124 can be repeated in order or, can be repeated in any desired order to produce the desired conditions on theelectrodes64,66 and68.

Referring to FIG. 6, the fourth state ofoperation120 occurs when switches76,82 and84 are closed, and switches78,80 and86 are open. Whenswitch76 is closed, current is applied toelectrode68 via connection ornode88. By closingswitches82 and84,electrodes64 and66 are connected tocommon return75 ofpower source62 through connection ornodes92 and90, respectively.

FIG. 16 illustrates the condition of theplasma chamber70 during the fourth state ofoperation120.Electrode68 comprises an anode, whileelectrodes64 and66 comprise cathodes.Plasma128 is generated proximate tocathode64. Ions fromplasma128 are attached towardscathode64, while electrons fromplasma128 are attracted towardsanode68. Aplasma130 is generated proximate tocathode66. Ions fromplasma130 are attracted tocathode66, while electrons fromplasma130 are attracted toanode68.

FIG. 14 illustrates thevoltage132 onelectrode68 during thefourth state120,fifth state122 andsixth state124. As shown in FIG. 14, thevoltage132 onelectrode68 is slightly positive during thefourth state120, and negative during thefifth state122 andsixth state124.

FIG. 17 illustrates the condition of theplasma chamber70 during thefifth state122. As shown,electrodes64 and68 comprise cathodes, whileelectrode66 comprises an anode. Aplasma134 is generated proximate tocathode64. Ions fromplasma134 are attracted tocathode64, while electrons fromplasma134 are attracted toanode66. Similarly, aplasma136 is generated proximate tocathode68. Ions fromplasma136 are attracted tocathode68, while electrons fromplasma136 are attracted toanode66.

FIG. 15 illustrates thevoltage138 onelectrode66 during thefourth state120,fifth state122 andsixth state124. During thefourth state120, thevoltage138 onelectrode66 is negative. During thefifth state122, thevoltage138 onelectrode66 is slightly positive. During thesixth state124, thevoltage138 onelectrode66 is negative.

Referring to FIG. 6, switches80,84 and86 are closed and switches76,78 and82 are open during the sixth state. As shown in FIG. 6, the direct current74 is applied toelectrode64 via connection ornode92. When switches84 and86 are closed,electrodes66 and68 are coupled to thecommon return75 ofpower source62 via nodes orconnections90 and88, respectively.

The condition of theplasma chamber70 during thesixth state124 is illustrated in FIG.18. As shown in FIG. 18,electrode64 comprises an anode, whileelectrodes66 and68 comprise cathodes. Aplasma140 is generated proximate tocathode68. Positive ions fromplasma140 are attracted tocathode68, while negative electrons are attracted toanode64. Similarly, aplasma142 is generated proximate tocathode66. Ions fromplasma142 are attracted tocathode66, while negative electrons are attracted toanode64. Although not shown, all of the switches of FIG. 6 can be closed during the same time to generate a seventh state of operation in which no current flows through theplasma chamber70. It is possible to operate the system of FIG. 6 in any combinations of these states depending upon desired results. This seventh state of operation, as described above, may be utilized for arc discharge dissipation or to provide a duty cycle within theplasma chamber70.

FIG. 19 is a schematic circuit diagram of another embodiment of the present invention. The embodiment of FIG. 19 illustrates the use of a single current controlledpower source144 that is capable of generating a substantially constant supply of direct current172 in combination with fourelectrodes146,148,150 and152 disposed in aplasma chamber154. FIGS. 6 and 19 illustrate the manner in which any desired number of electrodes can be placed in a single plasma chamber utilizing a singlecurrent control power source.Switches156,158,160,162,164,166,168 and170 can be opened and closed in any desired configuration to generate various states within theplasma chamber154 as desired.

The advantages of using multiple electrodes in a plasma chamber are that the target surfaces which comprise the cathode can be changed from one electrode to another to provide additional cathode surfaces. Moreover, additional anode surfaces are provided in theplasma chamber154 to attract negative electrons that enhances the generation of the plasma. Of course, any desired configuration of the electrodes can be used within theplasma chamber154 other than that shown in FIG. 19, or any of the figures.

FIG. 20 is a schematic circuit diagram of another embodiment of the present invention. As shown in FIG. 20, a current controlledpower source174 generates a substantially constant supply of direct current176. An additional current controlledpower source178 generates a substantially constant supply of direct current180.Switches182 and184 are connected in a shunt configuration withpower source174 andpower source178, respectively. Aplasma chamber190 is disposed in the circuit so that anelectrode186 is coupled to acommon return188 ofpower source174.Electrode192 is similarly connected to acommon return194 ofpower source178.Power source174 includes aninductor196 that assists thepower source174 in functioning as an ideal current source. Similarly,inductor198 ofpower source178 assists thepower source178 in functioning as an ideal current source.

In operation, the embodiment of FIG. 20 has three different operating states. In a first operating state,switch182 is closed and switch184 is open. Direct current176 frompower source174 is shunted to thecommon return188 and does not pass through theplasma chamber190. However, direct current180 frompower source178 passes throughswitch182 toelectrode186 inplasma chamber190. The direct current180 then passes through theplasma chamber190 toelectrode192 tocommon return194 ofpower source178. Hence, the direct current180 passes through theplasma chamber190 in a first direction fromelectrode186 toelectrode192.

FIG. 21 illustrates the flow of current through theplasma chamber190. As shown in FIG. 21, in a first state of operation, the direct current180 passes through theplasma chamber190 for a predetermined period whileswitch182 is closed and switch184 is open. In a second state of operation, switch182 is open and switch184 is closed. Direct current180 frompower source178 is shunted to thecommon return194 ofpower source178 and does not pass through the plasma ofplasma chamber190. However,direct current176 ofpower source174 flows through theswitch184 toelectrode192. Direct current176 flows fromelectrode192 toelectrode186 that is connected tocommon return188 ofpower source174. In this manner, direct current176 passes through theplasma chamber190 fromelectrode192 toelectrode186. As FIG. 21 shows, a pulse of direct current176 passes through theplasma chamber190 during a predetermined period whenswitch184 is closed and switch182 is open. FIG. 21 also shows a manner in which theswitches182 and184 can be alternately opened and closed to allow the direct current180 and direct current176 to alternately pass through theplasma chamber190 in a periodic fashion. FIG. 21 additionally illustrates that direct current176 and direct current180 are not necessarily equal. Of course, these direct currents frompower sources174 and178 can be generated at any desired magnitude that is consistent with the operation of theplasma chamber190. FIG. 21 simply illustrates that thedirect currents176 and180 need not necessarily be equal in magnitude.

FIG. 22 schematically illustrates the manner in which switches182 and184 can also be operated in a third state. As shown in FIG. 22, direct current180 passes through the plasma chamber whenswitch182 is closed and switch184 is open during a first state of operation. Bothswitches182 and184 can then be closed during apredetermined time period200 so that no current passes through theplasma chamber190 for example, whenever conditions are detected in the plasma chamber that could result in an arc discharge. Switch182 can then be opened and switch184 can remain closed so that direct current176 passes through theplasma chamber190, as illustrated in FIG.22. These states can be periodically repeated, as shown in FIG. 22, to produce a predetermined duty cycle of pulses that are applied to theplasma chamber190. Of course, any desired order of states can be applied by switching theswitches182 and184 in the positions to produce the desired state of operation.

FIG. 23 illustrates another alternative embodiment of the present invention that utilizes three power sources. Each of the power sources has an associated shunt switch and electrode. For example,power source202 has an associatedshunt switch208 andelectrode214 that are both coupled to thecommon return216 ofpower source202. Similarly,power source204 has an associatedshunt switch210 and anelectrode218 that is connected to acommon return220 ofpower source204.Power source206 has an associatedshunt switch212 andelectrode222 that are connected tocommon return224 ofpower source206. Various states of operation can be generated employing the embodiment of FIG. 23 similar to the various states of operation of the embodiments of FIGS. 7-15 with the exception that separate currents can be generated by each of thepower sources202,204 and206. For example, in one state of operation, switch208 is closed, whileswitches210 and212 are open. In that case, the substantially constant supply of direct current226 is shunted throughswitch208 tocommon return216 so that the substantially constant supply of direct current226 does not pass through theplasma chamber232. However, the substantially constant supply ofdirect currents228 and230 frompower sources204 and206, respectively, pass throughswitch208 and are applied toelectrode214. Direct current228 passes through plasma generated in theplasma chamber232 fromelectrode214 toelectrode218 tocommon return220 ofpower source204. In a similar manner, direct current230 passes fromelectrode214 through theplasma chamber232 toelectrode222, and to thecommon return224 ofpower source206. As can be seen, various states of operation can be generated by opening and closing theswitches208,210, and212 at various predetermined times. Of course, if all of the switches are closed, no current passes through theplasma chamber232.

FIG. 24 illustrates the manner in which fourpower sources234,236,238,240 can be employed with fourshunt switches242,244,246 and248 and four associatedelectrodes250,252,254 and256, respectively. The embodiment of FIG. 24 can be operated in a manner similar to that disclosed with respect to the operation of the embodiment of FIG.23. FIG. 24 also illustrates that any number of power supplies can be used in conjunction with a similar number of electrodes and shunt switches.

FIG. 25 is a schematic circuit diagram of another embodiment of the present invention. As illustrated in FIG. 25, apower source260 generates a supply of a substantially constant direct current262.Switches264 and266 are connected in parallel to theoutput268 of thepower source260. Aplasma chamber270 havingelectrodes272 and274 is connected tonodes276 and278 that are, in turn, connected toswitches264 and266, respectively.Inductors280 and282 are connected tonodes276 and278, and thecommon return284 ofpower source260.

In operation, the embodiment of FIG. 25 has several different states of operation. In startup mode, switch264 may be closed whileswitch266 is open. In this state of operation, current increases ininductor280 for a predetermined period. In a second state of operation, during the startup phase,switch264 is opened and switch266 is simultaneously closed so that the direct current262 flows tonode278.Inductor280 attempts to draw some of the current262 through theplasma chamber270 fromelectrode274 toelectrode272. Initially,inductor282 provides a certain amount of impedance so that all of the current262 cannot immediately flow through theinductor282 when theswitch266 is first closed. These factors, in combination, cause the plasma to ignite under normal conditions so that a flow of current is established in the plasma chamber fromelectrode274 toelectrode272. The current262, however, increases oninductor282 for a predetermined period. As the current increases oninductor282, the current throughplasma chamber270 andinductor280 decreases.Switch266 is then opened and switch264 is substantially simultaneously closed. At that point, current flows fromelectrode272 toelectrode274 to maintain the current ininductor282. In a similar manner, the current builds oninductor280 while the current lessens oninductor282 until theswitches264,266 are switched again. Of course, bothswitches264 and266 can be closed to prevent the flow of current in theplasma chamber270. Although the embodiment of FIG. 25 utilizesinductors280 and282 that have substantially equal inductances, it is possible that the embodiment of FIG. 25 can be operated with inductors that do not have the same impedance. Additionally, the operation ofswitches264 and266 is dependent upon the ramping time ofcurrent inductors280 and282, so that the switching period ofswitches264 and266, as well as the efficiency of the system, is dependent upon the magnitude of the inductance ofinductors280 and282. With longer switching periods, the current flow inplasma chamber270 may take the appearance of ramped pulses rather than square pulses.

FIG. 26 is a schematic illustration of another embodiment of the present invention that utilizes asingle power source286 that produces a substantially constant supply of direct current288 that is applied to aplasma chamber290 having threeelectrodes292,294 and296. Threeswitches298,300 and302 are coupled to thepower source286 for supplying the direct current288 toelectrodes292,294 and296, as well as toinductors304,306 and308, respectively. The embodiment of FIG. 26 operates in a manner similar to the embodiment of FIG. 25 by using the impedance ofinductors304,306 and308 to cause current to flow between theelectrodes292,294 and296 inplasma chamber290.

FIG. 27 illustrates another alternative embodiment of the present invention that utilizes asingle power sources310 that generates a substantially constant supply of direct current312 that is applied to aplasma chamber314 that has fourelectrodes316,318,320,322.Switches324,326,328 and330 are connected toinductors332,334,336 and338, andelectrodes316,318,320 and322, respectively, in a manner similar to that illustrated in FIG.26. FIG. 27 illustrates that the number of electrodes in aplasma chamber314 can be increased utilizing asingle power source310 by increasing the number of switches and inductors that are connected in the manner shown. The advantages associated with the use of multiple electrodes, as described above, can be realized with the embodiments of FIGS. 26 and 27 while employing only a single power source.

FIG. 28 discloses another embodiment to the present invention that is similar to the embodiment of FIG. 6 with the exception thatpower source340 comprises a voltage controlled power source. The embodiment of FIG. 28 operates in substantially the same manner as the embodiment of FIG. 6 with the exception that thepower source340 provides a substantially constant supply of voltage toelectrodes342,344 and346 ofplasma chamber348 by activation ofswitches350,352,354,356,358,360. As indicated with respect to FIG. 1, when the slope of the current verses voltage curve is less than 45 degrees, it may be advantageous to use a voltage controlled power source rather than a current controlled power source since incremental changes in the voltage will produce smaller incremental changes in the current. FIG. 29 illustrates the manner in which the number of electrodes in aplasma chamber364 can be increased utilizing a single voltage controlledpower source362. The embodiment of FIG. 29 is similar to the embodiment of FIG. 19 with the exception that thepower source362 is a voltage controlled power source. Theswitches366,368,370,372,374,376,378 and380 can be operated to apply voltages to thevarious electrodes382,384,386 and388 to produce a plasma inplasma chamber364.

The present invention therefore provides various embodiments for generating a plasma in a plasma chamber using current controlled power sources that are capable of accurately controlling the amount of current delivered to a plasma chamber. High temperature plasmas have low resistances such that slight changes in voltages cause large changes in the amount of current delivered to the plasma chamber. Excessive increases in current increase the susceptibility of arc discharges in the plasma chamber. Since the present invention utilizes a current controlled source in association with high temperature plasmas, the amount of current is regulated utilizing a power source that resembles a current source. The present invention encompasses embodiments employing a single power source with multiple electrodes in the plasma chamber, as well as embodiments including both multiple power sources an multiple electrodes. The present invention also encompasses various switching arrangements to produce various states of operation.

The foregoing discussion of the advantages of current sourcing relates to the short times involved in the pulsing operation and in arc formation, detection, and quenching. In reactive sputter deposition of certain oxides, the impedance of the plasma drops as the target sputtering region is encroached by oxide formation on the target. If, as is true for many materials, the oxide has a higher secondary emission coefficient than the metal itself, the plasma impedance will drop as the oxide encroaches. This is so because as ions strike the insulating surface, secondary electrons are emitted with will be pulled into the plasma and increase its density, lowering the target voltage to a given power level. This is another way of stating that the plasma impedance will drop. Thus, if the power supply is set up to hold the voltage constant on the target, the power (and therefore the sputtering rate) will increase as the voltage drops. This will increase the metal available to react with the background gas, and inhibit to some extent the encroachment of insulator on the sputter area. Inhibiting the encroachment will make the process more stable and easier to control.

To be effective as a stabilizing approach, the holding of the voltage constant must be on the time scale of the chemical reactions of the background oxygen (or other reactive gas) with the target and the deposited film, which is measured in milliseconds. The requirement to be current sourced is on a time scale of the arcs and the pulsing, which is measured in microseconds. That is, the power source should appear to be a constant current source for the interpulse period. To permit this duality of control, one must set up a current-sourced power supply to be voltage regulated, which means that the value of the instantaneous current is to be continuously adjusted by the regulation loops of the power supply to maintain the voltage constant on a millisecond time scale. By this means, on a short time scale, measured in microseconds, the current is held constant, while on a longer time scale, measured in milliseconds, the power supply appears to hold the voltage constant. Additionally, the use of multiple electrodes in association with a single voltage controlled power source is an alternative embodiment of the present invention that may provide advantages in low temperature plasma processes.

The foregoing description of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise from disclosed and other modifications and variations may be possible in light of the above teachings. For example, various embodiments disclosed in the present application may be utilized with a voltage controlled power source which may have advantages in low resistance plasma applications. The embodiment was chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and various modifications as are suited to the particular use contemplated. It is intended that the appended claims be construed to include other alternative embodiments of the invention except insofar as limited by the prior art.

Claims (9)

What is claimed is:

1. A circuit that causes at least two substantially constant currents to flow to at least two electrodes in a plasma chamber, comprising:

a first power source that generates a substantially constant supply of a first direct current, said first power source having an output lead coupled to a first electrode, and a common return lead coupled to a circuit common;

a second power source that generates a substantially constant supply of a second direct current, said second power source having an output lead coupled to a second electrode and a common return lead coupled to a circuit common;

a first switch coupled between said first electrode and said circuit common that, when closed, causes current to flow from said second power source through said plasma chamber from said second electrode to said first electrode and shunts current from said first power source to said circuit common;

a second switch coupled between said second electrode and said circuit common that, when closed, causes current to flow from said first power source in a second direction through said plasma chamber from said first electrode to said second electrode, and shunts current from said second power source to said circuit common.

2. The circuit of claim1 wherein:

at least one of said power sources includes a series connected inductor.

3. The circuit of claim1 wherein said first direct current and said second direct current are not equal.

4. A circuit for causing two substantially constant direct currents to flow in a plasma chamber comprising:

a first current controlled power source coupled to said plasma chamber that generates a first substantially constant direct current;

a second current controlled power source coupled to said plasma chamber that generates a second substantially constant direct current;

a first switch that causes current from said second current controlled power source to flow through said plasma chamber in a first direction when closed;

a second switch that causes current from said first current controlled power source to flow through said plasma in a second direction when closed.

5. The circuit of claim4 wherein:

said first current controlled power source includes a series connected inductor;

said second current controlled power source includes a series connected inductor.

6. The circuit of claim4 further comprising:

at least on additional current controlled power source that generates at least one additional substantially constant direct current;

an additional switch associated with each additional current controlled power source that causes current from other current controlled power sources to flow through said plasma chamber in an additional direction.

7. A method for causing two substantially constant direct currents to flow in a plasma chamber comprising the steps of:

generating a first substantially constant direct current using a first current controlled power source;

generating a second substantially constant direct current using a second current controlled power source;

connecting said first current controlled power source to said plasma chamber to cause said first substantially constant direct current to flow through said plasma chamber in a first direction during a first state of operation;

connecting said second current controlled power source to said plasma chamber to cause said second substantially constant direct current to flow through said plasma chamber in a second direction during a second state of operation.

8. The method of claim7 wherein said steps of generating first and second substantially constant direct currents comprises generating first and second substantially constant direct currents that are not equal in magnitude.

9. The method of claim7 further comprising the step of:

shunting said first and second substantially constant direct current from said first and second current controlled power sources to a common return so that no current passes through said plasma chamber whenever conditions are detected in said plasma chamber that could result in an arc discharge.

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